Risk based maintenance puts the maintenance effort where the risk is: each asset is scored on how likely it is to fail and how much it would matter, and the schedule is built from the top of that list down. It is the practical answer to a team with more assets than hours, and it is the reasoning behind most sensible schedules whether or not the words are used. This page says how the scoring is done, what the ranking decides, and what it looks like on a register of a few hundred assets.
Scoring consequence
For each asset, what happens if it fails: injury or environmental release, the line stops, a single machine stops, or nothing anyone notices for a week. Score it on a short scale, three or four levels is enough, and score it for the worst credible failure mode rather than the most common. Consequence is usually known by the people who run the operation and needs no measurement; it is the part of the score that is most often argued and least often wrong.
Scoring likelihood
How often the asset fails, from its reactive work order history if there is one, from the manufacturer's data or from the technicians' memory if there is not. Score on the same short scale. This is the part of the score the record improves: a year of work orders replaces a guess with a count, which is one reason to keep the history before doing the ranking rather than after.
The ranking and what it decides
Multiply or plot consequence against likelihood and the register sorts into a top corner, a bottom corner and a middle. Top corner: high consequence, high likelihood, these get the preventive tasks, the inspections and the spares first. Bottom corner: low on both, run to failure with a note on the register. Middle: a judgement per asset, usually an inspection interval rather than a replacement interval. The ranking is also the order for building the schedule when there is not time to do it all at once.
On a register of a few hundred assets
The scoring takes an afternoon with the supervisor and the senior technician, and the result is typically a few dozen assets that deserve most of the schedule, a hundred that need an inspection, and the rest that can be left to fail with a spare. This site's worked example, 180 assets with 60 preventive jobs a month, is a schedule of that shape. The maintenance log worksheet has a column for the asset's criticality so that the ranking travels with the record.
Questions people ask about risk based maintenance
Is risk-based maintenance the same as RCM?
RCM analyses each asset's failure modes in detail; risk-based maintenance ranks assets to decide which deserve that detail. Most sites do the ranking on everything and the RCM on the top corner.
How often should the scores be revisited?
When the history changes the likelihood, when the operation changes the consequence, and after any failure the ranking did not expect.
Can risk-based maintenance justify doing less?
Yes, on the bottom corner, and that is half its point. Effort removed from assets that do not matter is effort available for the ones that do.